The map sensors are pretty common, my VPC comes with a 3 bar(2 bar positive pressure, 1 bar vacuum). GM makes a 4 bar sensor, not sure what the part number is, best off to do a google search. They should be under $100 new, much cheaper used.
I know the same thing can be done with a pocketlogger program provided you can write your own parameters for a separate input. I had the info for the MMCD software, but haven't seen it yet for the pocketlogger software.
*edit* I'm sure you won't be going over 29 psi, so here's the GM part number for the 3-bar sensor. It was used on the 89 Turbo TA. part #12223861, retails for $51 on
www.gmpartsdirect.com